1. Field of the Invention
The invention relates to Radio Frequency Identification (RFID) Transponders and, more particularly, to an RFID transponder comprising a housing for mounting on or in a workpiece having a metallic surface.
2. Description of the Related Art
Mobile data memories that can be read contactessly are often used for identifying goods, machines and other articles. Here, optically readable barcode labels are increasingly being replaced by electromagnetically or magnetically writeable and readable radiofrequency identification (RFID) labels, i.e., RFID transponders. In preferred configurations, these RFID transponders operate without a dedicated power supply, such as a battery.
Different technologies are used in RFID transponders depending on the case of use. Thus, for labeling garments and other far field applications, for example, the RFID transponders are coupled to the reader by radio, i.e., by electromagnetic waves. However, inductively coupled RFID transponders will be considered below. These can be coupled to a writing and reading device, reader for short, by an alternating magnetic field and are often used in industrial applications in which short distances have to be bridged (i.e., near field detection).
Inductively coupled RFID transponders regularly have one or more transmitting and receiving coils by which the magnetic flux generated by a reader is converted into an AC voltage, where the AC voltage initially supplies the operating electronics of the RFID transponder with energy, and then contains information transmitted from the reader to the RFID transponder and information transmitted from the RFID transponder to the reader. With respect to the function of the RFID transponder, it is essential that the magnetic flux generated by the reader is substantially conducted through the area of the coil. However, this is problematic in cases in which the RFID transponder is operated on a metallic surface or in a depression in a metallic workpiece. In these cases, often a large portion of the magnetic flux is conducted through the metallic workpiece and laterally past the transmitting and receiving coil (or simply coil), where the energy of the alternating magnetic field is largely converted into heat by eddy current losses and is therefore no longer available for supplying the RFID transponder. Conversely, the emissions of the RFID transponder in the case of an arrangement on a metallic workpiece are also allowed to be conducted directly into the metallic workpiece, such that an undesirable attenuation can likewise be observed.
In order to solve the foregoing problem, it is known to provide between the transmitting and receiving coil of the RFID transponder and a metallic surface or a metallic workpiece a ferrite core (this also includes plate-shaped and pot-shaped components composed of ferrite material). As a result, the magnetic flux, i.e., the magnetic field lines, substantially coming from a receiving side opposite to the metallic workpiece, are conducted through the area of the transmitting and receiving coil and are thereafter conducted away from the metallic workpiece to the receiving side again.
However, the introduction of a ferrite core has disadvantages. Thus, the mounting of this component is an additional manufacturing step, where it is often necessary to provide separate spacers for spacing apart the ferrite core with respect to the metallic surface of the metallic workpiece. Ferrite cores furthermore have the disadvantage that they are very brittle and can therefore hardly be processed subsequently by a user. Consequently, ferrite cores usually have to be procured from a manufacturer in their finally required form, such that a subsequent change in the design of the RFID transponder, for example, changes in the dimensions of the coil or of a housing, have the effect that a new series of ferrite cores has to be designed, ordered, manufactured and supplied. The mounting of the ferrite cores is also associated with expenditure because there is the risk of cracks and fractures because of the brittle material. An already completed transponder having such a ferrite core is also sensitive to mechanical effects that can lead to cracks and fractures in the ferrite and thus to a deterioration in the maximum reading and receiving distance (range). Therefore, it is customary to encapsulate the transponders with ferrite cores in impact-resistant plastic housings which absorb the mechanical loads. However, this increases the structural size of the RFID transponders constructed in this way.